firewire: remove bogus check in fw_core_handle_request

This check is bogus:
  - Maximum asynchronous payload size for S800...S3200 is 4096.
  - The p->payload_length is totally uninteresting.  Only the
    request->length of the subsequently allocated and initialized
    struct fw_request is of significance.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
Signed-off-by: Kristian Høgsberg <krh@redhat.com>